The structures of free-standing platinum nanowires are systematically investigated by using genetic algorithm simulation with an EAM potential. The magnetic properties are also studied. Several helical multi-shell cylindrical and pentagonal packing structures are observed. These multi-shell structures are composed of coaxial atomic shells with the three and four strand helical, centered pentagonal, and hexagonal, and parallel double chain-core curved surface epitaxy. Under the same growth sequence, the numbers of atomic strands in inner and outer shells exhibit even-odd coupling. (C) 2003 Published by Elsevier B.V.